Washing clothing in general requires sufficiently penetrating the water in which a laundry detergent is dissolved into the structure of fabric in order to remove dirt and stains on the fabric. In order to increase the penetration of water, a conventional washing machine typically agitates water containing a detergent as well as clothing in a laundry tub using an agitator or impeller, or tumbles the clothing submerged in water in a laundry drum.
Further, some attempts have been made to adapt an ultrasonic cleaning technique for washing clothing. A washing machine utilizing an ultrasonic cleaning technique generates an ultrasonic wave in the water containing a detergent in a laundry tub to induce ultrasonic cavitation. The ultrasonic cavitation is known to create in a moment vacuum bubbles in a liquid which immediately and violently implode to produce millions of microscopic jets of fluid. Fabric of clothing submerged in water in a laundry tub is attacked by the implosions, thereby allowing dirt and stains to be removed from the fabric. The ultrasonic cavitation is generated at anti-nodes of an ultrasonic wave in the form of a standing wave. Since the implosion energy of the ultrasonic cavitation is too high for the fabric, the ultrasonic cavitation not only causes the dirt and stains on the fabric to be removed therefrom but also causes the fabric to be eroded, which is referred to as the cavitation erosion. In order to protect the fabric against the cavitation erosion, the ultrasonic standing wave may be swept so that the ultrasonic cavitation does not continuously occur at the same points.
In addition, in a circumstance where a bunch of clothing is jumbled up in a laundry tub, a standing wave field is difficult to be created, and thus the ultrasonic cavitation cannot be stably generated within the structure of the fabric, resulting in insufficient cleaning action.